Amplitude stabilized oscillator

ABSTRACT

An electrometer apparatus having an amplitude stabilized oscillator circuit with an electromechanical resonant transducer connected to the input of the oscillator active stage. The transducer has first and second terminals. A differentially connected operational amplifier is also provided having plus and minus inputs, the first terminal of the electromechanical resonant transducer being connected to the plus terminal of the operational amplifier, and the second terminal being connected to ground. A light emitting source is connected between the output of the operational amplifier and ground, and is responsive to the output of the operational amplifier to supply a corresponding amount of energy to a light responsive element connected to the output of the operational amplifier. A negative feedback path connected between the light responsive element and the minus terminal of the operational amplifier is also provided. The electrometer may be used to measure unknowns such as the electrostatic potential of a surface in non-contacting manner or an electrostatic field.

United States Patent [191 Vosteen et a1.

[ Jan. 8, 1974 1 1 AMPLITUDE STABILIZED OSCILLATOR Q 22 Filed: ..1uly6,1972

21 Appl. No.: 269,454

[52] US. Cl 331/183, 331/116, 318/132,

331/109 [51] Int. Cl. H031) 3/02 [58] Field of Search 331/141, 116,116M,

[56] References Cited UNITED STATES PATENTS 2,773,219 12/1956 Aron331/109 X 3,117,288 l/1964 Modiano 331/109 3,319,184 5/1967 McCall331/183 X 3,462,615 8/1969 Bernstein 323/21 UX 3,504,301 3/1970 Hetzel331/1 16 M 3,596,205 7/1971 Ribner 331/109 3,618,130 11/1971 Garuts,.331/109 3,663,894 5/1972 Hetzel 331/116 M 3,705,316 12/1972 Burrow eta1... 307/311 3,713,045 l/1973 Usuda et a1 331/183 OPERATlONALELECTRO-MECHANICAL Primary ExaminerD. F. Duggan Att0rney-A1bert J.Santorelli 5 7] ABSTRACT An electrometer apparatus having an amplitudestabilized oscillator circuit with an electromechanical resonanttransducer connected to the input of the oscillator active stage. Thetransducer has first and second terminals. A differentially connectedoperational amplifier is also provided having plus and minus inputs, thefirst terminal of the electromechanical resonant transducer beingconnected to the plus terminal of the operational amplifier, and thesecond terminal being connected to ground. A light emitting source isconnected between the output of the operational amplifier and ground,and is responsive to the output of the operational amplifier to supply acorresponding amount of energy to a light responsive element connectedto the output of the operational amplifier. A negative feedback pathconnected between the light responsive element and the minus terminal ofthe operational amplifier is also provided. The electrometer may be usedto measure unknowns such as the electrostatic potential of a surface innon-contacting manner or an electrostatic field.

12 Claims, 2 Drawing Figures REF. S1GNAL RESONANT TRANSDUCER (EMT)AMPLIFIER l llll OPTlONAL IMPEDANCE TRANSmRMER llll PATENIEDJAN ROPERATIONAL AMPLIFIER ELEfTRO-NECHAN/CAL REF. SI6NAL FEEITBZJ L l l 7 1AMPLITUDE STABILIZED OSCILLATOR BACKGROUND OF THE INVENTION 1. Field ofthe Invention The invention relates to an electrometer apparatus tostatic field, and employs an oscillator providing improved amplitudestability. The oscillator utilizes a two terminal electromechanicalresonant element connected to its active stage, which vibrates acapacitor de tector to produce a modulated signal representative of theunknown being measured.

2. Description of the Prior Art The use of electrometer apparatus tomeasure unknowns such as the electrostatic potential of a surface innoncontacting manner or an electrostatic field is known in the art. NoteU.S. Pat. No. 3,611,127 which employs a transducer to vibrate acapacitor detector at a predetermined frequency to produce a modulatedsignal representative of the unknown being measured. It is also known inthe prior art that for purposes of efficiency, it may be desirable tooperate an electromechanical transducer at its natural mechanicalresonant frequency in order to produce a maximum amplitude ofoscillation with the least drive power for the trans ducer.

If a separate oscillator is used to excite the transducer at resonance,various influences can cause a shift between the excitation frequencyand the natural resonant frequency of the transducer. It is thennecessary to provide a frequency adjustment in order to insuresatisfactory operation. In the event of a shift in resonant frequency onthe part of either the transducer or its excitation source, asignificant phase shift can occur between the excitation signals and themechanical displacement of the transducer, which may be objectionablefor certain applications. For example, the oscillator may be used as areference oscillator to excite the transducer whereby the mechanicaldisplacement thereof produces modulated signals, and may also supplyreference signals for application to a phase sensitive detector toeffect demodulation of modulated signals, as in US. Pat. No. 3,611,127.Under such circumstances wherein a phase sensitive detector is utilizedfor phase sensitive or synchronous rectification, a fixed phaserelationship between the reference signals and the mechanicaldisplacement of the transducer is required.

It is thus preferable to employ the transducer toalso function as theresonant element of the oscillator, in order to eliminate the need forfrequency adjustments and provide a stable phase relationship betweenthe electrical excitation signal of the oscillator and the mechanicaldisplacement of the transducer.

SUMMARY OF THE DISCLOSURE The invention relates to an oscillator thatmay be used in electrometer apparatus, having an electromechanicalresonant transducer which functions as the resonant element of theoscillator to provide a constant amplitude excitation signal to thetransducer that is substantially purely sinusoidal. The excitationsignal thus provides constant conversion gain for the modulation actionof the transducer, and the substantially sinusoidal waveform permitsstraight forward balancing out of any residual in-phase or quadratureunbalance in the modulated signal.

The oscillator according to the invention utilizes a two terminalresonant circuit such as a magnetic or PM dynamic transducer. Typically,mechanical resonance of the transducer occurs and is reflected into theelectrical circuit of the oscillator as an electrical impedance which ismaximized at resonance. One of the terminals of the two terminalresonant circuit is grounded, eliminating the necessity associated withmost oscillators of requiring at least one additional terminal forfeedback purposes. The grounding of one terminal of the two terminalresonant circuit permits a single, grounded coaxial shielded lead tointerconnect the transducer and its associated active circuitry thusproviding excellent and simple shielding between the excitation signalsand any low level signal which might be carried in a sharedmulti-conductor cable.

The active device of the oscillator comprises a differentially connectedoperational amplifier having a light dependent resistance network whichis responsive to the output of the operational amplifier to provide aconstant amplitude excitation signal, having a waveform that issubstantially purely sinusoidal. A light emitting device is alsoconnected to the output of the operational amplifier to control thelight dependant resistance network.

The electrometer apparatus may also include a wide band, non-invertingamplifier output stage, utilizing a photoemitter and light sensitiveresistance means arrangement.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an electronic schematicdiagram of the oscillator circuit according to the invention; and

HO. 2 illustrates the manner in which the oscillator circuit of FIG. llmay be used in an electrometer apparatus.

DETAILED DESCRIPTION OF THE DISCLOSURE The oscillator circuit accordingto the invention is shown in FIG. 11. The active stage comprises asingle differential input, single-ended output operational amplifier 1.A bridge network having passive linear resistors R11 and R3,electromechanical transducer 2, and light dependent resistance R2 isutilized. Thus the output of operational amplifier 11 may be tracedthrough resistor R1 and transducer 2 to ground, and through re sistanceR2 and resistor R3 to ground. The common connection of resistance R2 andresistor R3 is connected to the minus input of operational amplifier l;to provide a negative feedback path which is utilized for amplitudestabilization purposes. The common connection of resistor RT andelectromechanical transducer 2 is connected to the plus input ofoperational amplifier l.

-A full-wave bridge rectifier network comprising diodes Dl, D2, D3 andD4 is connected in series with resistor R4 between the output ofoperational amplifier 1 and ground. The full-wave bridge rectifiernetwork feeds the series connection of light emitting diode D5 and zenerdiode D6, the described series connection being connected at one end tothe common connection of diodes D1 and D3, and at the other end to thecommon connection of diodes D2 and D4. The light emitting diode D5 isoptically coupled to light dependent resistance R2. The full-wave bridgerectifier network is thus driven by the output of operational amplifier1.

At the instant of energization of the oscillator circuit, correspondingto zero oscillator amplitude, light emitting diode D is not energizedand the resistance of light dependent resistance R2 effectively presentsan open circuit to the output of the operational amplifier 1 because itis not illuminated by diode D5. Consequently the negative feedback pathfor operational amplifier l is not complete and the oscillator amplitudeincreases. As the operational amplifier 1 output voltage, which is analternating current voltage, increases such that it is sufficient tobias zener diode D6 to conduction, the light emitting diode D5 isenergized and emits light which is coupled to dependent resistance R2.This causes light dependent resistance R2 to conduct and limit theamplitude of the oscillator at the output of the operational amplifier 1to a stable, low distortion sinusoidal output voltage.

The light emitting diode D5 is thus activated by biasing zener diode D6to conduction. The full-wave bridge rectifier network causes lightemitting diode D5 to emit short bursts of light at twice the oscillatorfrequency. However, light dependent resistance R2 has a natural speed ofresponse which is slow in comparison to onehalf of the period of theoscillator. Consequently light dependent resistance R2 exhibits arelatively stable resistance and insures a low distortion sinusoidaloscillator output. The particular light dependent resistance and lightemitting diode utilized should be selected to insure that the spectraloutput of the light emitting diode adequately matches the spectralresponse curve of the light dependent resistor. An incandescent lightcould be substituted for the full-wave bridge network shown, however itsspeed-of-response is comparable to that of the light dependentresistance with the result that oscillator amplitude instability canoccur in the amplitude stabilizing feedback loop. In contradistinctionthe light emitting diode D5 exhibits instantaneous response thus causingthe light dependent resistance R2 to exhibit the only significant lag inthe amplitude regulating feedback loop.

Resistor R4 is connected in series with the full-wave rectifier networkto primarily control the duty cycle of the pulse current feeding thelight emitting diode D5. Otherwise, a high momentary peak load on theoperational amplifier output could cause overloading and clipping.Capacitor Cl. shunts the electromechanical transducer 2 in order toswamp or subdue undesired high frequency oscillation componentsresulting from spurious electrical resonances occurring at frequencieswell above the natural mechanical resonant frequency of the system. Thisswamping action is effected by the connection of capacitor C1 in shuntrelation with the electromechanical transducer 2 without adverselyinfluencing the desired resonant frequency output.

The optional impedance transformer 3 may be connected between thecoaxial cable and the electromechanical transducer. This may benecessary if an odd impedance transducer is employed to insure properdrive power within the output voltage and current capability ofoperational amplifier 1.

When the operational amplifier l is operating, the ratio [RI/12(2)][RZ/RS] of the respective resistance values of elements comprising thebridge network is satisfied.

FIG. 2 shows the manner in which the oscillator circuit described aboveand shown in FIG. 1 may be used in an electrometer apparatus of the typedescribed in US. Pat. No. 3,61 1,127. FIG. 3 of this patent illustratesan electrostatic voltmeter and FIG. 4 shows electrostatic fieldmeter.With respect to FIG. 2 of the instant application, transducer 2 isutilized to drive sensitive electrode 18 and is connected to oscillator14 in the manner described with reference to FIG. 1. The probe assembly7 of FIG. 2 is similar to that described in patent 3,6l 1,127 andcomprises bottom plate 10 having aperture 1 l, sensitive electrode 18and preamplifier 16. The outputs of signal amplifier l2 and referenceoscillator 14 are connected to phase sensitive detector 13. Aphotoemitter/photoconductor network 19 is connected to the output ofoperational amplifier l5 and operates in the manner described in US.Pat. No. 3,590,251 to provide a wide band, non-inverting amplifier atthe output of the electrometer apparatus. This configuration canfunction as a voltage follower having a bandwidth permitting a signal togo through which is wider than the carrier, providing thephotoconductors have sufficient speed-of-response. The series connectionof R-5 and C-2 is connected between the summing junction of operationalamplifier l5 and ground to provide feedback.

It is noted that the ground designation shown in FIG. 1 for theoscillator circuit would be the same connection as circuit common inFIG. 2, when the oscillator circuit of FIG. 1 is employed as referenceoscillator 14 of the electrometer system shown in FIG. 2.

We claim:

1. An amplitude stabilized oscillator circuit comprising:

an electromechanical resonant transducer having first and secondterminals,

a differentially connected operational amplifier having plus and minusinputs, the first terminal of the electromechanical resonant transducerbeing connected to the plus terminal of the operational amplifier, thesecond terminal being connected to ground,

a light emitting source connected between the output of the operationalamplifier and ground, and responsive to the output of the operationalamplifier to supply a corresponding amount of energy to a lightresponsive element connected to the output of the operational amplifier,and

a negative feedback connection connected between the light responsiveelement and the minus terminal of the operational amplifier.

2. An amplitude stabilized oscillator circuit as recited in claim llwherein the light emitting source comprises a full-wave rectifiernetwork and a light emitting device driven thereby.

3. An amplitude stabilized oscillator circuit as recited in claim '2wherein the light responsive element and the transducer are connectedwith passive resistance elements to form opposite arms of a bridgenetwork.

4. An amplitude stabilized oscillator circuit as recited in claim 2wherein the full-wave rectifier network comprises a diode bridge networkfeeding a light emitting device comprising the series connection of alight emitting diode and a zener diode.

5. An amplitude stabilized oscillator circuit as recited in claim 3wherein the full-wave rectifier network comprises a diode bridge networkfeeding a light emitting device comprising the series connection of alight emitting diode and a zener diode.

6. An amplitude stabilized oscillator circuit as recited in claim 5wherein the light responsive element comprises a photoconductive device.

7. An amplitude stabilized oscillator circuit as recited in claim 6wherein the transducer comprises a magnetic transducer.

8. An amplitude stabilized oscillator circuit as recited in claim 1wherein the light responsive element comprises a photoconductive device.

9. An amplitude stabilized oscillator circuit as recited in claim 1further comprising a single, grounded coaxial shielded lead connectingthe transducer to the opera tional amplifier.

10. In an electrometer apparatus to measure unknowns such as theelectrostatic potential of a surface in noncontacting manner or anelectrostatic field, having a capacitive detector positionable inelectrostatic coupling relationship with the surface or in theelectrostatic field to produce a detector signal representative of themagnitude and polarity of the unknown being measured for long-termstatic measurement, the improvement comrising:

an electromechanical resonant transducer having first and secondterminals, operative to vibrate the capacitive detector at apredetermined frequency to vary the coupling relationship and producemodulated detector signals having a carrier frequency equal to thepredetermined frequency,

an amplitude stabilized reference oscillator circuit to producereference signals at the predetermined frequency,

a differentially connected operational amplifier having plus and minusinputs, the first terminal of the electromechanical resonant transducerbeing connected to the plus terminal of the operational amplifier, thesecond terminal being connected to ground,

a light emitting source connected between the output of the operationalamplifier and ground, and responsive to the output of the operationalamplifier to supply a corresponding amount of energy to a lightresponsive element connected to the output of the operational amplifier,

a negative feedback connection connected between the light responsiveelement and the minus terminal of the operational amplifier, and

a detector connected to receive the reference signals and modulateddetector signals at a fixed phase relationship to demodulate the latterand produce an output signal indicative of the magnitude and po larityof the unknown being measured.

11. The electrometer apparatus recited in claim 10 further comprising awide ban, non-inverting voltage follower connected to the output of thedetector having an operational amplifier and a photoelectric networkconnected to pass a frequency range wider than the carrier frequency.

12. The electrometer apparatus recited in claim 10 further comprising awide band, non-inverting voltage follower connected to the output of thedetector.

l= l l UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent3,784,930 Dated January 8, 1974 "Inventor(s) Robert E. Vosteen et a1.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Cancel claims 10 through 12. On the cover sheet "12 Claims" should read9 Claims I Signed and sealed. this 15th day of April 1975.

(SEAL) Attest:

C. ZIARSHALL DANN Commissioner of Patents and Trademarks RUTH C. I-{ASONAttesting Officer F ORM PO-IOSO (10-69) UsCOMM-DC 603764"? u.s.aovnuuim- PRINTING orncz: 93 o UNITED STATES OFFICE CERTIFICATE OFCORRECTION Patent No. 3,784,930 Dated January 1974 inventor) Robert E.Vosteen et a1.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Cancel claims 10 through 12. On the cover sheet "12 Claims" should readv9 Claims Signed and sealed. this 15th day of April 1975.

(SEAL Attest:

C HAP-SHALL DANE? RUTH C. E-iASON Commissioner of Patents AttestingOfficer and Trademarks FORM PO-IOSO (10-69) USCOMM-DC 60376'P69 U.S.GQVERNNEHT PRINTING OFFICE a 9. 9 o

1. An amplitude stabilized oscillator circuit comprising: anelectromechanical resonant transducer having first and second terminals,a differentially connected operational amplifier having plus and minusinputs, the first terminal of the electromechanical resonant transducerbeing connected to the plus terminal of the operational amplifier, thesecond terminal being connected to ground, a light emitting sourceconnected between the output of the operational amplifier and ground,and responsive to the output of the operational amplifier to supply acorresponding amount of energy to a light responsive element connectedto the output of the operational amplifier, and a negative feedbackconnection connected between the light responsive element and the minusterminal of the operational amplifier.
 2. An amplitude stabilizedoscillator circuit as recited in claim 1 wherein the light emittingsource comprises a full-wave rectifier network and a light emittingdevice driven thereby.
 3. An amplitude stabilized oscillator circuit asrecited in claim 2 wherein the light responsive element and thetransducer are connected with passive resistance elements to formopposite arms of a bridge network.
 4. An amplitude stabilized oscillatorcircuit as recited in claim 2 wherein the full-wave rectifier networkcomprises a diode bridge network feeding a light emitting devicecomprising the series connection of a light emitting diode and a zenerdiode.
 5. An amplitude stabilized oscillator circuit as recited in claim3 wherein the full-wave rectifier network comprises a diode bridgenetwork feeding a light emitting device comprising the series connectionof a light emitting diode and a zener diode.
 6. An amplitude stabilizedoscillator circuit as recited in claim 5 wherein the light responsiveelement comprises a photoconductive device.
 7. An amplitude stabilizedoscillator circuit as recited in claim 6 wherein the transducercomprises a magnetic transducer.
 8. An amplitude stabilized oscillatorcircuit as recited in claim 1 wherein the light responsive elementcomprises a photoconductive device.
 9. An amplitude stabilizedoscillator circuit as recited in claim 1 further comprising a single,grounded coaxial shielded lead connecting the transducer to theoperational amplifier.
 10. In an electrometer apparatus to measureunknowns such as the electrostatic potential of a surface innoncontacting manner Or an electrostatic field, having a capacitivedetector positionable in electrostatic coupling relationship with thesurface or in the electrostatic field to produce a detector signalrepresentative of the magnitude and polarity of the unknown beingmeasured for long-term static measurement, the improvement comrising: anelectromechanical resonant transducer having first and second terminals,operative to vibrate the capacitive detector at a predeterminedfrequency to vary the coupling relationship and produce modulateddetector signals having a carrier frequency equal to the predeterminedfrequency, an amplitude stabilized reference oscillator circuit toproduce reference signals at the predetermined frequency, adifferentially connected operational amplifier having plus and minusinputs, the first terminal of the electromechanical resonant transducerbeing connected to the plus terminal of the operational amplifier, thesecond terminal being connected to ground, a light emitting sourceconnected between the output of the operational amplifier and ground,and responsive to the output of the operational amplifier to supply acorresponding amount of energy to a light responsive element connectedto the output of the operational amplifier, a negative feedbackconnection connected between the light responsive element and the minusterminal of the operational amplifier, and a detector connected toreceive the reference signals and modulated detector signals at a fixedphase relationship to demodulate the latter and produce an output signalindicative of the magnitude and polarity of the unknown being measured.11. The electrometer apparatus recited in claim 10 further comprising awide ban, non-inverting voltage follower connected to the output of thedetector having an operational amplifier and a photoelectric networkconnected to pass a frequency range wider than the carrier frequency.12. The electrometer apparatus recited in claim 10 further comprising awide band, non-inverting voltage follower connected to the output of thedetector.